The present invention relates to inkjet printing apparatus and is concerned, more particularly, with the purging of air from a permanent inkjet cartridge base after a new ink supply has replaced a previous ink supply.
In a thermal ink jet printer, energy pulses, in a printhead, are used to heat and vaporize ink in an ink channel formed in the printhead. This vaporized ink creates vapor bubbles that grow to fill the channels and expel ink that was in the channel out through a set of orifices on the face of the printhead. The orifices are shaped to direct the ink outward onto a recording medium. Ink then refills the channel inside the printhead, usually by capillary action, which in turn draws ink into the printhead from an ink supply.
It is usually necessary to prime a printhead of an inkjet printer before use, to remove air and ensure that the printhead is full of ink. Priming, for example, may be carried out by applying suction to the ink ejecting orifices to draw ink into the printhead from the ink supply under pressure. Some approaches use a suction cap over the orifices to recover the ink ejection function of the printhead after a period of non-use. In many such printers, the printhead receives its supply of ink from a sub-receptacle, which in turn receives its supply of ink from a main receptacle. Air collects in the sub-receptacle and is removed by applying suction to the sub-receptacle before suction is supplied to the printhead orifices. The mechanism providing the suction adds additional cost and complexity to the printer.
Some printers reduce complexity by just using a replaceable ink cartridge that incorporates a pressure regulating mechanism within the ink supply. The pressure regulating mechanism inside the ink cartridge increases the size and cost of the cartridge body. The increased size of the cartridge body in turn requires a greater carriage mass and cost, thereby discouraging production of more compact, portable, and low-priced inkjet printers. A significant number of improvements in printheads and pressure regulator mechanisms have occurred over the years. These improvements are now yielding improvement in the useful life of printheads and pressure regulators which exceeds the expected use of the supply of ink. Thus, when the user discards the ink cartridge, the printhead and pressure regulating mechanisms are also discarded, even though they may have a significant period of usable life remaining. Disposal of these parts, which may still be useful, and any remaining ink in the ink supply results in an increased cost to the user and is an inefficient use of resources.
To address problems with disposable ink cartridges, some inkjet printers have permanent, refillable remote ink supplies that are not mounted to the carriage. Such ink supplies, because they are stationary within the printer, are not subject to all of the size constraints of an ink supply that is moved with the carriage. Usually, the printhead will include a small ink reservoir that is periodically replenished by moving the printhead to a refilling station that has a stationary built in reservoir. See, for example, commonly assigned U.S. Pat. No. 4,968,998.
Other printers use replaceable remote reservoirs that are not located on the carriage and do not move with the printhead during printing. Replaceable reservoirs are often plastic bags filled with ink. The bag is provided with a septum that can be punctured by a hollow needle, for coupling ink inside the bag to the printer and which allows ink to flow from the bag to the printhead. The bag may be squeezed or pressurized in some other manner to cause ink to flow from the reservoir.
An example of an inkjet printing system using ink reservoirs is disclosed in U.S. Pat. No. 5,650,811. In this system, ink is drawn from a stationary reservoir and pressurized to propel the ink through a supply tube to a first ink containment receptacle in an ink jet cartridge mounted on a movable printer cartridge. A regulator mechanism within the cartridge body intermittently opens to supply ink to a second ink containment receptacle in the cartridge that couples with the printhead orifices through a sub-receptacle which contains an ink filter. The regulator maintains a sufficient backpressure on the ink to prevent it from drooling out of the printhead.
All of these different printer systems are plagued by unwanted air that enters the ink reservoir, supply lines and cartridge in a variety of ways. Air is primarily introduced into the system by evolving as gas when ink is heated at the printhead. Other sources of air ingestion are from empty supply lines before printer initialization, by "air gulping" through fluid interconnects during start up or operation and by diffusion through the walls of system components (such as cartridge body walls or tubes). Air in the ink supply system can cause "dry firing" of the drop generator (usually a resistor or piezo electric actuator), which damages the printhead. Alternatively, gas bubbles in supply lines can interfere with hydraulic flow through supply lines or capillary movement of ink through the small orifices at the printhead.
Printer vendors have tried many different approaches to eliminate unwanted gas in the ink supply flow path. Some inkjet cartridges (such as the cartridges used in the HP 2000C inkjet printer available from Hewlett-Packard Co.) are designed with empty internal space to "warehouse" air over the life of the cartridge. The additional space required for warehousing air in the moveable cartridge increases the size of the printer to accommodate the bulky cartridge over its path of movement. Another vendor's printer uses an air separator between an ink reservoir and cartridge body to remove air from the ink supply flow path, but this solution requires complex additional components that increase the size and cost of the printer.
Therefore, a new ink supply system is needed that can be replaceable, yet allow for the use of a permanent printhead, and which allows for production of low cost and non-complex printers than existing designs.